This invention relates generally to lasers, and more particularly, to cascaded lasers.
As is known in the art, a laser is a device that produces optical radiation using a population inversion to provide Light Amplifcation by Stimulated Emission of Radiation and, generally, an optical resonant cavity to provide positive feedback. An injection laser diode (ILD) or more simply a diode laser, is a laser employing a forward-biased semiconductor junction for the active medium.
As is also known, series-coupled diode lasers are capable of producing RF link gain, while retaining voltage, incremental resistance, and slope efficiency properties that are the sum of the individual lasers. Both series and in-line arrays of discrete devices have parasitic capacitance and inductance resulting from interconnections between the devices. These parasitic effects limit the performance of the array. The series devices suffer the further problem of being difficult to couple into fiber. A solution to both problems is to series couple separate laser structures through Esaki or back diodes during the epitaxial process. This type of device is known as a bipolar cascade laser (BCL).
Ideally, the BCL operates by having each injected electron participate in a recombination event in the topmost laser junction then tunnel from the valence band of the first junction into the conduction band of the next junction, participate in another recombination event, and so on through each stage of the cascade. This is illustrated in FIGS. 1 and 1A. In this way the quantum wells (QW) of the laser junctions are coupled in series, as opposed to the parallel filling of conventional multi-QW devices. For N cascaded gain sections each injected electron can produce up to N photons. This cascading effect is the source of the improved differential slope efficiency cascade lasers enjoy over conventional (parallel coupled) multiple quantum well lasers. Since each injected electron produces several photons, the bipolar cascade laser provides signal gain.
Conventional edge-emitting bipolar cascade lasers have been of two varieties. In the first, the cascaded gain sections and tunnel junctions have been placed inside of a single dielectric waveguide. This structure has the advantage of increasing the overlap of the field with the active regions (known as xcex93) thereby ideally reducing the threshold current of the device approximately proportionally to the number of cascaded active regions. The inclusion of the highly doped, and hence highly absorbing, tunnel junctions in the waveguiding region also substantially increases optical loss for the fundamental mode, negating part or all of the increase in xcex93.
An alternative design is to have separate active/waveguiding regions, as in traditional edge-emitting designs, each coupled electrically during the epitaxial process via a tunnel junction (FIG. 1). While no improvement in xcex93 is realized with this approach, the optical absorption losses are minimally effected by the presence of the tunnel junction. One problem with this approach, however, is that the separate waveguide design lies in its generated beam profile. The preferred lasing mode is xe2x80x9codd.xe2x80x9d That, is the field in each waveguide is 180 degrees out of phase with the adjacent waveguides. The result of this excitation is a beam profile with an on-axis null which prevents efficient coupling into single-mode optical fiber.
An xe2x80x9cevenxe2x80x9d field mode (i.e. a mode in which the field in each waveguide is in-phase with the adjacent waveguides) can be generated by increasing the distance between the waveguides (thereby decoupling them). Such a large separation, however, further exacerbates efficient single-mode coupling. While the series coupling of the active regions is preferably accomplished electrically, it is desirable to electromagnetically couple the waveguides in parallel.
As is also known, optical waveguiding in diode lasers is normally achieved by sandwiching a first dielectric material having a first optical index of refraction between two xe2x80x9ccladdingxe2x80x9d layers of dielectric material. Each of the cladding layers are provided having an optical index of refraction which is lower than the index of refraction of the first dielectric material. Light incident upon the interface from the high index side at angles greater than the critical angle results in total power reflection. By placing an active gain material, e.g. a quantum well, in the high index region a large overlap is achieved between the optical field and the active region, resulting in stimulated emission or xe2x80x9clasing.xe2x80x9d
A similar problem with an on-axis farfield null, as described above, has been encountered in the field of high-power diode laser arrays. To circumvent this problem, an anti-reflecting resonant optical waveguiding (ARROW) structure can be used. In this design, the active regions are placed inside the low optical index material. Normally this would cause the optical fields generated by the active medium to be scattered. By proper selection of the dimensions of the high and low index regions of the anti-guide the lateral component of the field is made to resonate, however. In this way the entire structure acts as a single waveguide. The inclusion of a pair of appropriately dimensioned anti-reflectors at either end of the resonant structure ensures the fields inside the lateral resonator retain the proper phase relationship for emitting most of the optical power into a single on-axis farfield lobe.
When employed in a high-power diode laser array, the ARROW guiding structure was implemented in the lateral direction through multiple processing steps and epitaxial regrowth. The use of standard semiconductor processing techniques limits the minimum achievable dimensions and index contrast. Using epitaxial growth techniques, the ARROW can be implemented in the vertical direction. The advantages of epitaxy also include monolayer control over the device dimensions afforded by modern growth technologies such as MBE and MOCVD. Further, varying index contrasts can be achieved by using the full range of lattice matched ternary and quaternary materials achievable through MBE and MOCVD. The ability to precisely control the optical index, active region properties, and dimensions in each section of the ARROW independently during the epitaxial growth process permits the output near and farfield patterns to be tailored to specific application needs.
Aside from the beam profile and implementation disadvantages inherent in the lateral ARROW, an additional disadvantage arises in that individual active regions must be electrically driven in parallel.
It would, therefore, be desirable to provide a device which allows transmission of signals having a relatively high signal to noise ratio over an optical fiber.
In accordance with the present invention, a bipolar cascade-ARROW laser includes a bipolar cascade laser and an anti-resonant reflecting optical waveguide (ARROW). The bipolar cascade laser electrically couples multiple active regions during the epitaxial process via highly doped p-n junctions known as tunnel junctions. The ARROW is a means of electromagnetically parallel-coupling multiple active sections, while minimizing lateral radiation loss. With this particular arrangement, a laser which allows transmission of signals having a relatively high signal-to-noise ratio transmission over optical fiber is provided. The electrical coupling of the active regions inside a single active region results in the laser of the present invention having reduced noise and improved SNR. Unlike conventional semiconductor lasers, in the laser of the present invention multiple photons are generated for every injected electron. This enables the laser of the present invention to produce higher signal levels without a commensurate increase in noise. Furthermore, unlike existing approaches to cascade lasers, the laser of the present invention emits light at wavelengths suitable for long distance fiber optic transmission with a beam profile that is compatible with efficient coupling into optical fiber. Thus, the device is suitable for communication and other applications.
Furthermore, the optical loss introduced by the highly doped tunnel junctions serves the useful purpose of improving mode discrimination between the desired fundamental optical mode and higher order optical modes rather than interfering with the operation of the bipolar cascade-ARROW laser. Moreover, use of an electrical cascade via highly doped tunnel junctions to obtain differential quantum efficiencies in excess of 100% while use of an ARROW structure permits optical coupling of multiple active regions. A further advantage is that the BCL ARROW laser of the present invention has a diffraction limited optical beam of narrow divergence angle as a result of the optical coupling of the low index active regions.
Since the bipolar cascade-ARROW laser can be realized in many material systems, such as the alloys of In, Ga, As, P, Al, N, Sb, and in virtually all present configurations including but not limited to Fabry-Perot, distributed feedback, distributed Bragg reflectors, alpha and vertical cavity surface emitting lasers, it can be used in analog and digital communications applications, as well as in medical applications. Furthermore the laser of the present invention finds use in any application which requires high power and/or high power array applications. The bipolar cascade-ARROW laser of the present invention is also superior to other diode lasers in that the output beam couples more efficiently into optical fiber, the noise is lower, the electrical to optical conversion is more efficient, and the highly absorbing regions of the device improve, rather than diminish, performance (i.e. the optical loss introduced by the highly doped tunnel junctions serves the useful purpose of improving mode discrimination between the desired fundamental optical mode and higher order optical modes). Thus, as listed above, the bipolar cascade-ARROW laser of the present invention embodies superior performance over conventional lasers in many important performance criteria.
In summary, the use of the bipolar cascade laser in combination with the vertical ARROW structure enables the ARROW laser (or the bipolar cascade-ARROW laser) to be implemented vertically during the epitaxial process. Previous versions of ARROW lasers have been implemented laterally by using areas of regrowth to introduce the necessary index variation, with the active regions being electrically pumped in parallel. Combining the BCL-ARROW laser with the prior art of lateral ARROW lasers leads to full two dimensional arraying.
The advantages and improvements of the bipolar cascade-ARROW laser over conventional devices includes: (1) use of an electrical cascade via highly doped tunnel junctions to obtain differential quantum efficiencies in excess of 100%; (2) use of an ARROW structure to permit optical coupling of multiple active regions; (3) a device having a diffraction limited optical beam of narrow divergence angle; (4) a device having reduced noise and improved SNR resultant from electrical coupling of the active regions inside a single active region; and (5) a device in which the optical loss introduced by the highly doped tunnel junctions serves the useful purpose of improving mode discrimination between the desired fundamental optical mode and higher order optical modes rather than interfering with the operation of the bipolar cascade-ARROW laser.